A proton exchange membrane fuel cell, also known as a polymer electrolyte membrane (PEM) fuel cell (PEMFC) converts electrochemical energy released during the hydrogen and oxygen electrode reactions in to electrical energy. A stream of hydrogen is delivered to the anode side of the membrane electrode assembly (MEA). The half-cell reaction at the anode, the hydrogen oxidation reaction (HOR), splits hydrogen in to protons and electrons. The newly generated protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the MEA, thus creating the current output of the fuel cell. Meanwhile, a stream of oxygen (typically in air) is delivered to the cathode side of the MEA. At the cathode side, oxygen molecules are reduced by the electrons arriving through the external circuit and combine with the protons permeating through the polymer electrolyte membrane to form water molecules. This cathodic half-cell reaction is an oxygen reduction reaction (ORR). Both half-cell reactions are typically catalyzed by platinum based materials. Each cell produces about 1.1 volt, so to reach the desired voltage for a particular application the cells are combined to produce stacks. The cells are separated by bipolar plates which also provide a hydrogen fuel distribution channel, as well as providing a method of extracting the current. PEM fuel cells are considered to have the highest energy density of all the fuel cells, and due to the nature of the reactions, have the quickest start up time (less than 1 second). Therefore, they tend to be favored for applications such as vehicles, portable power, and backup power applications.
A PEM fuel cell operating in an automotive application typically undergoes thousands of start-up/shut-down events over multiple years of operation. During these transient periods of repeated fuel cell start up/shut down cycles, and also during other abnormal fuel cell operation conditions (e.g., a cell reversal caused by local fuel starvation), the electrodes can be driven temporarily to relatively high positive potentials, significantly beyond their normal operational values and beyond the thermodynamic stability of water (i.e., >1.23 volt). These transient high potential pulses can lead to degradation of the catalyst layer. Corrosion of the carbon support can also occur for carbon supported catalysts.
Incorporation of oxygen evolution reaction (OER) catalysts to favor water electrolysis over carbon corrosion or catalyst degradation/dissolution is a relatively new material-based strategy for achieving fuel cell durability during transient conditions by reducing cell voltage. Ru has been observed to exhibit excellent OER activity but it is preferably stabilized. Ir is well known for being able to stabilize Ru, while Ir itself has been observed to exhibit good OER activity.
Before start-up, the anode flow field is typically filled with air. During the fuel cell start-up, the gas switches from air to hydrogen, resulting in an Hz-air front that moves through the anode flow field. When the fuel cell is shut-down, an Hz-air front formed by the gas switching moves through the anode flow field in the reverse direction. It is known that hydrogen and oxygen within the moving Hz-air front can recombine and produce water, especially when a catalyst such as platinum is present. This reaction can be relatively violent.
It is desirable to reduce the negative effects of the gas switching on the MEA performance.